The invention concerns a process for the manufacture of self-binding pigmentary particles, dry or in aqueous suspension or dispersion, containing at least one inorganic matter and at least one binder.
Inorganic particles and the polymer binders are two of the essential components used in the manufacture of various coatings; the inorganic particles, generally calcium carbonate, contribute to the final product certain properties such as optical properties, and the binders, generally in the form of latex suspensions or dispersions, give an overall cohesion to all the constituent elements used in the process to manufacture coating compositions.
However, it has been known for many years that complex interactions may occur between the calcium carbonate and all the forms of binders, causing the final properties of the coating products to be modified. For example, “Physical and chemical modifications in latex binders and their effect on the coating colour rheology” (Advanced Coating Fundamentals Symposium, San Diego, Calif., United States, May 4-5, 2001, pp. 108-123) concerns the influence of the physical and chemical modifications in a dispersion of styrene-butadiene binders on the rheological properties of a wet pigmentary coating composition.
For a long time the production of self-binding pigmentary particles has been sought: solid particles containing a polymeric binder and an inorganic pigment which act as a single material, thus preventing the end user from losing specifications due to undesirable interactions in various applications.
It was initially advanced that such self-binding pigments differ from the well-known pigmentary granulates through the nature of the forces assuring cohesion of these composite particles.
For example, document WO 01/00712 presents dry pigmentary granulates comprising 5% to 99% by weight of organic polymer pigment, from 0% to 94.5% by weight of inorganic pigment and from 0.5% to 5% by weight of a binder or mixture of binders.
Such granulates are manufactured by mixing all the constituents in dispersion and by drying by spraying of the said dispersion.
In addition, document WO 01/00713 describes plastic polymer pigments, applicable in a dry form, which can be hollow or solid, containing a polymer, a binder and an inorganic pigment.
As the inventors mention themselves, the conglomerates obtained consist of individual spherical particles bound together by electrostatic forces.
According to a different viewpoint, the concept of self-binding pigments must be considered in this document as mutually bound individual particles, in which the binding mechanisms use not only electrostatic forces but also chemical bindings created through sorption chemistry mechanisms, or physical bindings such as acid-base interactions between the inorganic matter and the polymeric binders and Lifhshitz/van der Waals interactions.
In accordance with this concept, document WO 93/11183 describes a process to prepare stable aqueous dispersions by forming two aqueous suspensions of inorganic matter and polymeric latex particles, by adjusting the Zeta potential of the suspensions, and finally by mixing the two suspensions. The Zeta potential may be modified through the use of an additive allowing the surface charge to be adjusted. The polymeric latex binder is supposed to be adsorbed strongly on the inorganic matter particles.
In addition, document WO 93/12183 teaches a process for the preparation of aqueous dispersions of composite particles comprising a latex and an inorganic matter, used in paints and coatings. The said polymeric latex particles act as a binder, contributing the necessary spacing between the inorganic particles to give the end product opacity and the brightness. The process is characterized in that the latex prepared by polymerisation in emulsion is mixed in an aqueous suspension of inorganic matter, which suspension of inorganic matter is pre-dispersed with poly(meth)acrylic acid or its salts, or copolymer polyelectrolytes of (meth)acrylic acid or its salts. The authors suppose that the dispersant of the pigment does not cover the entire surface of the inorganic matter, and that at least one portion of the said surface is available for the adsorption of the polymeric latex particles.
Finally, document U.S. Pat. No. 4,025,483 describes a process to increase the stability of titanium dioxide aqueous suspensions, by mixing the latter with polymeric latex particles. According to the examples, the polymeric latex particles increase the effectiveness of the dispersing agent which is contained in the initial inorganic matter suspension.
In the light of the prior art, the pigment-polymer particles can be obtained according to various processes which always make reference to the use of a third constituent, such as a dispersing agent or an additive allowing the surface charge to be adjusted, to formulate the initial aqueous suspensions which contain the inorganic matter on the one hand, and the polymeric compounds on the other, and which are later mixed. Introducing a third constituent may have the result of modifying drastically the end properties of the coating in which the pigment-polymer particles are incorporated.